Gas turbine engine

ABSTRACT

Fan containment system fitting around an array of radially extending fan blades mounted on a hub in an axial gas turbine engine. The fan containment system includes a fan case having an annular casing element for encircling the array of fan blades and a hook projecting in a radially inward direction from the annular casing element and positioned axially forward of the array of fan blades when the fan containment system fitted around fan blades. An annular fan track liner positioned substantially coaxial to the annular casing element. Clamping arrangement connects fan track liner to the hook. Clamping arrangement is configured under the condition that a fan blade impacts the fan track liner, the clamping arrangement releases connection between the hook and a portion of fan track liner so that a portion of the fan track liner can move towards the annular casing element to encourage the fan blade to impact the hook.

FIELD OF INVENTION

The invention relates to a fan containment system, a casing assembly, afan and/or a gas turbine engine.

BACKGROUND

Turbofan gas turbine engines (which may be referred to simply as‘turbofans’) are typically employed to power aircraft. Turbofans areparticularly useful on commercial aircraft where fuel consumption is aprimary concern. Typically a turbofan gas turbine engine will comprisean axial fan driven by an engine core. The engine core is generally madeup of one or more turbines which drive respective compressors viacoaxial shafts. The fan is usually driven directly off an additionallower pressure turbine in the engine core.

The fan comprises an array of radially extending fan blades mounted on arotor and will usually provide, in current high bypass gas turbineengines, around seventy-five percent of the overall thrust generated bythe gas turbine engine. The remaining portion of air from the fan isingested by the engine core and is further compressed, combusted,accelerated and exhausted through a nozzle. The engine core exhaustmixes with the remaining portion of relatively high-volume, low-velocityair bypassing the engine core through a bypass duct.

To satisfy regulatory requirements, such engines are required todemonstrate that if part or all of a fan blade were to become detachedfrom the remainder of the fan, that the detached parts are suitablycaptured within the engine containment system.

The fan is radially surrounded (or circumscribed) by a fan casing. It isknown to provide the fan casing with a fan track liner and a containmentsystem designed to contain any released blades or associated debris.Often, the fan track liner can form part of the fan containment system.

The fan track liner typically includes an annular layer of abradablematerial which surrounds the fan blades. During operation of the engine,the fan blades rotate freely within the fan track liner. At maximumspeed the blades may cut a path into this abradable layer creating aseal against the fan casing and minimising air leakage around the bladetips during cruise. Further incursions can occur during gusts or takeoff rotations over time.

Swept fan blades are increasingly used in turbofan engines as they offersignificant advantages in efficiency over conventional fan blades. Sweptfan blades have a greater chord length at their central portion thanconventional fan blades. This greater chord length means that ice thatforms on a swept fan blade follows the same rearward and outward path ason a conventional fan blade but may reach a radially outer tip of theblade before it reaches the trailing edge. The ice will therefore beshed from the blade tip and may strike the fan track liner within theblade off zone (that is the region where a blade would contact the fantrack liner in the event of a blade being detached from the fan).

A fan track liner used with a swept fan blade is therefore required tobe strong enough to resist ice impact whilst allowing a detached fanblade to penetrate and be contained therewithin.

In recent years there has been a trend towards the use of lighter fanblades, which are typically either of hollow metal or of compositeconstruction. These lighter fan blades have similar impact energy perunit area as an ice sheet, which makes it more difficult to devise acasing arrangement that will resist the passage of ice and yet notinterfere with the trajectory of a released fan blade.

A conventional fan containment system or arrangement 100 is illustratedin FIG. 1 and surrounds a fan comprising an array of radially extendingfan blades 40. Each fan blade 40 has a leading edge 44, a trailing edge45 and fan blade tip 42. The fan containment arrangement 100 comprises afan case 150. The fan case 150 has a generally frustoconical orcylindrical annular casing element 152 and a hook 154. The hook 154 ispositioned axially forward of an array of radially extending fan blades40. A fan track liner 156 is mechanically fixed or directly bonded tothe fan case 150. The fan track liner 156 may be adhesively bonded tothe fan case 150. The fan track liner 156 is provided as a structuralintermediate to bridge a deliberate gap provided between the fan case150 and the fan blade tip 42.

The fan track liner 156 has, in circumferential layers, an attritionliner 158 (also referred to as an abradable liner or an abradablelayer), an intermediate layer which in this embodiment is a honeycomblayer 160, and a septum layer 162. The septum layer 162 acts as abonding, separation, and load spreading layer between the attritionliner 158 and the honeycomb layer 160. The honeycomb layer 160 may be analuminium honeycomb. The tips 42 of the fan blades 40 are intended topass as close as possible to the attrition liner 158 when rotating. Theattrition liner 158 is therefore designed to be abraded away by the fanblade tips 42 during abnormal operational movements of the fan blade 40and to just touch during the extreme of normal operation to ensure thegap between the rotating fan blade tips 42 and the fan track liner 156is as small as possible without wearing a trench in the attrition liner158. During normal operations of the gas turbine engine, ordinary andexpected movements of the fan blade 40 rotational envelope causeabrasion of the attrition liner 158. This allows the best possible sealbetween the fan blades 40 and the fan track liner 156 and so improvesthe effectiveness of the fan in driving air through the engine.

The purpose of the hook 154 is to ensure that, in the event that a fanblade 40 detaches from the rotor of the fan 12, the fan blade 40 willnot be ejected through the front, or intake, of the gas turbine engine.During such a fan-blade-off event, a released fan blade 40 continues totravel in the direction of travel at the point of release, sosubstantially travels tangentially to a circumference defined byrotation of the fan. Impact with the containment system (including thefan track liner 156) of the fan case 150 prevents the released fan blade40 from travelling any further in a radially outward direction relativeto the circumference defined by rotation of the fan. The fan blade 40will also move forwards in an axial direction. This axially forward andradially outward motion results in the fan blade 40 colliding with thehook 154. Thus the fan blade 40 is held by the hook 154 and furtheraxially forward movement is prevented. A trailing blade (not shown) thenforces the held released blade rearwards where the released blade iscontained. Thus the fan blade 40 is unable to cause damage to structuresoutside of the gas turbine engine casings.

As can be seen from FIG. 1, for the hook 154 to function effectively, areleased fan blade 40 must penetrate the attrition liner 158 in orderfor its forward trajectory to intercept with the hook. If the attritionliner 158 is too hard then the released fan blade 40 may notsufficiently crush the fan track liner 156.

However, the fan track liner 156 must also be stiff enough to withstandthe rigours of normal operation without sustaining damage. This meansthe fan track liner 156 must be strong enough to withstand ice and otherforeign object impacts without exhibiting damage for example. Thus thereis a design conflict, where on one hand the fan track liner 156 must behard enough to remain undamaged during normal operation, for examplewhen subjected to ice impacts, and on the other hand allow the tip 42 ofthe fan blade 40 to penetrate the attrition liner 158. It is a problemof balance in making the fan track liner 156 sufficiently hard enough tosustain foreign object impact, whilst at the same time, not be so hardas to alter the preferred hook-interception trajectory of a fan blade 40released from the rotor. Ice that impacts the fan casing rearwards ofthe blade position is resisted by a reinforced rearward portion 164 ofthe fan track liner.

An alternative fan containment system is indicated generally at 200 inFIG. 2. The fan containment system 200 includes a fan track liner 256.The fan track liner 256 has, in circumferential layers, an attritionliner 258, an intermediate layer 260, and a septum 262. The fan trackliner 256 that is connected to the fan casing 250 at both an axiallyforward position and an axially rearward position. At the axiallyforward position, the fan track liner 256 is connected to the casing athook 254 via a fastener 266 that is configured to fail at apredetermined load. In the event of a fan blade detaching from theremainder of the fan, the fan blade impacts the fan track liner 256, thefastener 266 fails and the fan track liner pivots about a rearward pointon the fan track liner. Such an arrangement is often referred to as atrap door arrangement. The trap door arrangement has been found to helpbalance the requirements for stiffness of the fan track liner with therequirements for resistance of operational impacts (e.g. ice impacts)ensuring a detached blade is held within the engine.

SUMMARY OF INVENTION

The present invention seeks to improve the integrity of a fancontainment system. The structural integrity of the hook is important toimprove the reliability of the fan containment system in a fan blade offevent.

One way to improve the structural integrity of the hook would be toincrease the thickness of the hook and/or make the hook from a stiffermaterial, but both these changes would undesirably increase the weightof the fan containment system. Furthermore, both these changes mayaffect the performance of the containment system in a fan blade offscenario and/or there may be limited space in the containment system tomake changes to the hook design.

In a first aspect the invention provides a fan containment system forfitment around an array of radially extending fan blades mounted on ahub in an axial gas turbine engine. The fan containment system comprisesa fan case having an annular casing element for encircling an array offan blades and a hook projecting in a generally radially inwarddirection from the annular casing element and positioned axially forwardof an array of fan blades when the fan containment system is fittedaround said fan blades. An annular fan track liner is positionedsubstantially coaxial to the annular casing element. A clampingarrangement connects the fan track liner to the hook. The clampingarrangement is configured such that under the condition that a fan bladeimpacts the fan track liner, the clamping arrangement substantiallyreleases the connection between the hook and a portion of the fan trackliner so that at least a portion of the fan track liner can move towardsthe annular casing element so as to encourage the released fan blade toimpact the hook.

The use of a clamping arrangement mitigates the risk of the integrity ofthe hook being damaged when the fan blade impacts the fan track liner,so that the likelihood of containment of a released fan blade isimproved. Unlike conventional methods of connecting the fan track linerto the hook, the clamping arrangement does not require holes to beformed in the hook to receive a fastener, and as such the risk of thehook cracking is reduced. Reducing the risk of the hook crackingmitigates the risk of a fan blade not being contained in a fan blade offevent and also, in a worst case scenario, mitigates the risk of crackingfrom the fastener hole propagating to the main body of the casing (e.g.the annular casing element).

Tests carried out on selected embodiments have found that the clampingarrangement does not interfere with the movement of the fan track linertowards the annular casing element in a fan blade off scenario, andvibration integrity is maintained during normal operation.

The clamping arrangement may connect a leading edge of the fan trackliner to the hook. The clamping arrangement may be configured to limitradial movement of the fan track liner in both a radially inward andradially outward direction. The clamping arrangement may be a componentdistinct from the hook.

The clamping arrangement may comprise a first member radially opposing asecond member, the hook being positioned (or sandwiched) between thefirst member and the second member. For example, the clampingarrangement may comprise a clamp having a first jaw and a second jaw,the hook being clamped between the first jaw and the second jaw.

At least a portion of a component of the clamping arrangement may beconfigured to fail (e.g. deform, shear or fracture) when a fan bladeimpacts the fan track liner.

The clamping arrangement may comprise a first clamping member positionedradially outward of the hook and a second clamping member positionedradially inward of the hook, the hook being clamped between the firstclamping member and the second clamping member. The area of overlapbetween the second clamping member and the hook may be selected tocontrol the timing of when at least a portion of the fan track linerpanel moves towards the annular casing element in a fan blade offscenario.

The clamping arrangement may comprise a connector connecting the firstclamping member and the second clamping member to the fan track liner.

The second clamping member may be configured to fail (e.g. fracture,deform or shear) when a fan blade impacts the fan track liner.Alternatively, in embodiments where the clamping arrangement comprises aconnector connecting the first clamping member and the second clampingmember to the fan track liner, the connector may be configured to fail(e.g. deform, shear or fracture) when a fan blade impacts the fan trackliner.

The fan track liner may comprise a tray proximal to the annular casingelement. The fan track liner may comprise an intermediate layerconnected (e.g. bonded) to the tray. The fan track liner may comprise anattrition layer proximal, in use, to the fan blades. The fan track linermay comprise a septum layer substantially radially between theintermediate layer and the attrition layer.

The first clamping member may comprise a portion of the septum layerand/or the tray.

In exemplary embodiments, the connector that connects the first clampingmember and second clamping member to the fan track liner may be part ofthe fan track liner. For example, the connector may be defined by theseptum layer.

The hook may comprise an undercut that accommodates at least a portionof the second clamping member. Provision of an undercut can reduce anysteps in the gas washed surface of the fan containment system, and/orhelp to ensure that any steps are radially outward in an axial flowdirection so as to reduce disturbance to airflow along the gas washedsurface of the fan containment system.

The first clamping member may be connected to the second clamping membervia one or more fasteners (e.g. a bolt and a nut, the nut may have ananti-rotation feature). The one or more fasteners may be spaced from thehook. Spacing the fasteners from the hook means that the fastener doesnot interfere with the hook.

The fastener may extend through at least a portion of the fan trackliner (e.g. through the attrition layer and/or the septum layer and/orthe intermediate layer and/or the tray).

Recesses may be formed in a radially outer surface of the fan trackliner to accommodate the fastener. For example, the recesses may becut-away sections or depressions. Provision of such recesses can reduceor eliminate the protrusion of the fasteners from a nominal profile ofthe fan track liner so as to limit any impact of the fasteners on theoperation of the fan containment system in a fan blade off scenario.

An insert may be provided in the fan track liner for receiving thefastener. For example, the insert may extend through the radialthickness of the fan track liner, e.g. the insert may have a thicknesssubstantially equal to the intermediate layer, septum layer and/or theattrition layer. For example, the insert may extend to the tray of thefan track liner. The thickness and dimensions of the insert may beselected to further improve vibration integrity.

The first clamping member may comprise a plate bonded to at least aportion of the fan track liner, for example using an adhesive such as anepoxy resin. In embodiments where the fan track liner comprises anintermediate layer that has a honeycomb structure, at least a portion ofthe honeycomb structure may be filled with adhesive, e.g. epoxy resin,in the region of the fastener so as to provide additional strength andsealing.

The first clamping member may comprise at least a portion of a componentof the fan track liner.

The first clamping member may comprise a series of discretecircumferentially spaced plates. Additionally or alternatively, thesecond clamping member may comprise a series of discretecircumferentially spaced plates.

The discrete circumferentially spaced plates may be considered to betabs.

The spacing between the plates and/or the circumferential thickness ofthe plates and/or the radial thickness of the plates may be selected totune the load required to cause at least a portion of the fan trackliner to move towards the annular casing element.

Filler material may be provided in any gaps or recessed areas,particularly gaps on a gas washed surface of the fan containment system.For example, filler material may be provided between the second clampingmember and the hook (e.g. in a recess defined between an undercut of thehook and a forward face of the second clamping member), and/or over aradially inner surface of the second clamping member.

The first clamping member may comprise a substantially annular plate.Additionally or alternatively, the second clamping member may comprise asubstantially annular plate.

The first clamping member may comprise a substantially annular plateprofiled in an axial direction so as to form a series of waves.Additionally or alternatively, the second clamping member may comprise asubstantially annular plates profiled in an axial direction so as toform a series of waves.

The first and second clamping members may be manufactured from acomposite material or a metal material.

The second clamping member may include a radial portion that receives afastener and a plate portion that abuts against the hook, wherein theradial portion has a greater radial thickness that the plate portion.

The clamping arrangement may include a plate having a lip seated on aradially inward surface of the hook. For example, the second clampingmember may include a lip abutting a radially inward surface of the hook.

The fan track liner may have a trap door arrangement. For example, thefan track liner may be pivotally connected to the fan case such that atleast a forward portion of the fan track liner pivots towards the fancase when a fan blade impacts the fan track liner.

A voidal region may be provided between at least a forward portion ofthe fan track liner and the annular casing element. The voidal regionmay accommodate movement of the fan track liner when a fan blade impactsthe fan track liner.

The fan track liner may be defined by a plurality of fan track linerpanels. Each fan track liner panel may be connected to the hook by theclamping arrangement.

The clamping arrangement may include a plurality of pairs of first andsecond clamping members (each pair including one first clamping memberand one second clamping member), and one or more of the pairs of firstand second clamping members may connect each fan track liner panel tothe hook.

The clamping arrangement may comprise at least one pair of cooperatingwedge-shaped shims positioned between the clamping members and axiallyadjacent the hook. The shims may be arranged relative to one another todefine a radial spacer between the clamping members.

The shims of each said pair may be arranged relative to one another suchthat the radial spacer which they define has a radial dimension which isless than the local radial thickness of the adjacent hook.

Said radial dimension of each radial spacer defined by the cooperatingshims may be effective to position the second clamping member such thata radially innermost surface of the second clamping member isaerodynamically flush with a radially innermost surface of the hookand/or a radially innermost surface of the fan track liner when the hookis clamped between the first and second clamping members.

The system may have a plurality of said pairs of cooperating shims, saidpairs of shims being circumferentially spaced-apart around the fan case.

Each pair of shims may be substantially identical to each other pair ofshims.

The shims of each pair may be arranged relative to one anotherindependently of the shims of each other pair, such that the radialthickness of each radial spacer defined by a respective pair of shims isset independently to thereby accommodate any circumferential variationin the radial thickness of the hook.

The shims of each said pair may be each circumferentially tapered, e.g.the shims are tapered so as to have a radial thickness which variesalong their circumferential extent.

The shims of each said pair may be configured for circumferentialmovement relative to one another prior to the hook being clamped betweenthe clamping members, to thereby adjust the radial thickness of theradial spacer defined by the shims.

The shims of each pair each may define a respective contact surface,said contact surfaces being arranged in contact with one another.

The contact surfaces may be arcuate.

The contact surfaces may be roughened.

The shims of each pair may be adhesively bonded together to define arespective radial spacer.

The shims of each pair may be secured to one another by at least onefastener to define a respective radial spacer.

The second clamping member may extend circumferentially around at leasta circumferential region of the fan containment system.

In a second aspect the invention provides a fan containment system forfitment around an array of radially extending fan blades mounted on ahub in an axial gas turbine engine. The fan containment system comprisesa fan case having an annular casing element for encircling an array offan blades and a hook projecting in a generally radially inwarddirection from the annular casing element and positioned axially forwardof an array of fan blades when the fan containment system is fittedaround said fan blades. The fan containment system comprises an annularfan track liner positioned substantially coaxial to the annular casingelement. A clamp is provided, the clamp having a first jaw opposing asecond jaw. The hook is clamped between the first and second jaws of theclamp. The clamp is connected to the fan track liner or defines at leasta portion of the fan track liner, such that under normal runningconditions the clamp substantially fixes the radial position of the fantrack liner with respect to the hook, and under the condition that a fanblade impacts the fan track liner, the second jaw is configured to failso that at least a portion of the fan track liner can move towards theannular casing element so as to encourage the released fan blade toimpact the hook.

As will be appreciated by the person skilled in the art, any one, or anycombination, of the optional features of the fan containment system ofthe first aspect are also applicable to the fan containment system ofthe second aspect.

In a third aspect the invention provides a fan containment system forfitment around an array of radially extending fan blades mounted on ahub in an axial gas turbine engine. The fan containment system comprisesa fan case having an annular casing element for encircling an array offan blades and a hook projecting in a generally radially inwarddirection from the annular casing element and positioned axially forwardof an array of fan blades when the fan containment system is fittedaround said fan blades. An annular fan track liner is positionedsubstantially coaxial to the annular casing element. A clamp isprovided, the clamp having a first jaw opposing a second jaw. The hookis clamped between the first and second jaws of the clamp. The clamp isconnected to the fan track liner by a connector configured to fail at apredetermined load such that under normal running conditions the clampsubstantially fixes the radial position of the fan track liner withrespect to the hook, and under the condition that a fan blade impactsthe fan track liner the connector fails so that at least a portion ofthe fan track liner can move towards the annular casing element so as toencourage the released fan blade to impact the hook.

As will be appreciated by the person skilled in the art, any one, or anycombination, of the optional features of the fan containment system ofthe first aspect are also applicable to the fan containment system ofthe third aspect.

According to a fourth aspect of the present invention, there isprovided: a fan containment system for fitment around an array ofradially extending fan blades mounted on a hub in an axial gas turbineengine, the fan containment system comprising: a fan case having anannular casing element for encircling an array of fan blades, and a hookprojecting in a generally radially inward direction from the annularcasing element and positioned axially forward of an array of fan bladeswhen the fan containment system is fitted around said fan blades; anannular fan track liner positioned substantially coaxial to the annularcasing element; and a clamping arrangement connecting the fan trackliner to the hook, the clamping arrangement comprising a first clampingmember positioned radially outward of the hook and a second clampingmember positioned radially inward of the hook such that the hook isclamped between the first clamping member and the second clampingmember, the clamping arrangement being configured such that under thecondition that a fan blade impacts the fan track liner, the clampingarrangement substantially releases the connection between the hook and aportion of the fan track liner so that at least a portion of the fantrack liner can move towards the annular casing element so as toencourage the released fan blade to impact the hook, wherein theclamping arrangement further comprises at least one pair of cooperatingwedge-shaped shims positioned between the clamping members and axiallyadjacent the hook, the shims being arranged relative to one another todefine a radial spacer between the clamping members.

As will be appreciated by the person skilled in the art the fancontainment system of the fourth aspect may have any one or anycombination, of the optional features of the fan containment system ofthe first aspect.

A fifth aspect of the invention provides a casing assembly comprisingthe fan containment system of any one of the first, second, third orfourth aspects.

A sixth aspect of the invention provides a fan comprising the fancontainment system of any one of the first, second, third or fourthaspects.

A seventh aspect of the invention provides a gas turbine enginecomprising the fan containment system according to the first, second,third or fourth aspect.

DESCRIPTION OF DRAWINGS

The invention will now be described, by way of example only, withreference to the accompanying drawings in which:

FIG. 1 illustrates a partial view of a cross-section through a typicalfan case arrangement of a gas turbine engine of the prior art;

FIG. 2 illustrates a partial view of a cross-section through analternative fan case arrangement of a gas turbine engine of the priorart;

FIG. 3 illustrates a cross-section through the rotational axis of ahigh-bypass gas turbine engine;

FIG. 4 illustrates a partial cross-section through a fan bladecontainment system;

FIG. 5 illustrates a perspective view of a partial cross-section of thefan blade containment system of FIG. 4;

FIG. 6 illustrates a perspective view of a partial cross-section througha clamping arrangement of the fan blade containment system of FIG. 4;

FIG. 7 illustrates a perspective view from a radially outer surface of apartial cross-section through a clamping arrangement of a fan trackliner and clamping arrangement of the fan blade containment system ofFIG. 4;

FIG. 8 illustrates a perspective view from a radially outer surface of afan track liner and clamping arrangement of the fan blade containmentsystem of FIG. 4;

FIG. 9 illustrates a perspective view from a radially outer surface of afan track liner and clamping arrangement of an alternative fancontainment system; and

FIG. 10A illustrates a partial plan view of a second clamping member ofthe clamping arrangement of the fan containment system of FIG. 4;

FIGS. 10B and 10C illustrate a partial plan view of a second clampingmember of a clamping arrangement of alternative fan containment systems;

FIGS. 11 to 13 illustrate a partial cross section of further alternativefan containment systems;

FIG. 14 illustrates a partial cross-section through a yet furtheralternative fan containment system;

FIG. 15 illustrates a partial transverse cross-section through thecontainment system illustrated in FIG. 14, taken along line V-V in FIG.14;

FIG. 16 shows part of the fan containment system of FIG. 14 in moredetail; and

FIG. 17 illustrates a perspective view from a radially outer surface ofa fan track liner and clamping arrangement of the fan containment systemof FIG. 14.

DETAILED DESCRIPTION

With reference to FIG. 3 a bypass gas turbine engine is indicated at 10.The engine 10 comprises, in axial flow series, an air intake duct 11,fan 12, a bypass duct 13, an intermediate pressure compressor 14, a highpressure compressor 16, a combustor 18, a high pressure turbine 20, anintermediate pressure turbine 22, a low pressure turbine 24 and anexhaust nozzle 25. The fan 12, compressors 14, 16 and turbines 20, 22,24 all rotate about the major axis of the gas turbine engine 10 and sodefine the axial direction of the gas turbine engine.

Air is drawn through the air intake duct 11 by the fan 12 where it isaccelerated. A significant portion of the airflow is discharged throughthe bypass duct 13 generating a corresponding portion of the enginethrust. The remainder is drawn through the intermediate pressurecompressor 14 into what is termed the core of the engine 10 where theair is compressed. A further stage of compression takes place in thehigh pressure compressor 16 before the air is mixed with fuel and burnedin the combustor 18. The resulting hot working fluid is dischargedthrough the high pressure turbine 20, the intermediate pressure turbine22 and the low pressure turbine 24 in series where work is extractedfrom the working fluid. The work extracted drives the intake fan 12, theintermediate pressure compressor 14 and the high pressure compressor 16via shafts 26, 28, 30. The working fluid, which has reduced in pressureand temperature, is then expelled through the exhaust nozzle 25generating the remainder of the engine thrust.

The intake fan 12 comprises an array of radially extending fan blades 40that are mounted to the shaft 26. The shaft 26 may be considered a hubat the position where the fan blades 40 are mounted. FIG. 3 shows thatthe fan 12 is surrounded by a fan containment system 300 that also formsone wall or a part of the bypass duct 13.

In the present application a forward direction (indicated by arrow F inFIG. 3) and a rearward direction (indicated by arrow R in FIG. 3) aredefined in terms of axial airflow through the engine 10.

Referring now to FIGS. 4 to 6, the fan containment system 300 is shownin more detail. The fan containment system 300 comprises a fan case 350.The fan case 350 includes an annular casing element 352 that, in use,encircles the fan blades (not shown in FIGS. 4 to 6, but indicated at 40in FIG. 3) of the gas turbine engine (indicated at 10 in FIG. 3). Thefan case 350 further includes a hook 354 that projects from the annularcasing element in a generally radially inward direction. The hook 354 ispositioned, in use, axially forward of the fan blades and the hook isarranged so as to extend axially inwardly, such that in a fan blade offscenario the hook 354 prevents the fan blade from exiting the enginethrough the air intake duct (indicated at 11 in FIG. 3).

In the present embodiment, the hook 354 is substantially L-shaped andhas a radial component extending radially inwards from the annularcasing element 352 and an axial component extending axially rearwardtowards the fan blades from the radial component.

A fan track liner 356 is provided. The fan track liner 356 is annularand is positioned substantially coaxial to and radially inward of theannular casing element 352. A rearward end of the annular casing elementis connected to the fan case 350 using methods known in the art. The fantrack liner 356 includes a tray 357 to which an intermediate layer 360is connected (e.g. bonded), in this embodiment the intermediate layer isa honeycomb layer. An attrition layer 358 is positioned, in use,proximal to the fan blades 40. A septum layer 362 provides an interfacebetween the attrition layer and the intermediate layer, forming part ofthe bond between the two. The septum layer 362 also separates theattrition layer and the intermediate layer and distributes any appliedload between the attrition layer and the intermediate layer.

The fan track liner 356 is spaced radially inward from the casingelement 352 so that a voidal region 359 is formed between the fan trackliner 356 and the casing element 352.

In the present embodiment, the fan track liner is formed of a pluralityof adjacent arcuate panels arranged to be substantially coaxial. Forexample, in some installations it is envisaged that there might beprovided a total of sixteen fan track liner panels in abutting relationaround the fan case.

A forward end of the fan track liner 356 is connected to the hook 354via a clamping arrangement 370. Various configurations of clampingarrangements will be described, but in general, the clamping arrangementincludes a first clamping member 374 and second clamping member 372(e.g. the clamping arrangement includes a clamp having a radially innerand radially outer jaw) that clamp against a radially inner and radiallyouter surface of the hook.

In the embodiment shown in FIGS. 3 to 8, the clamping arrangement 370includes a plurality of first and second clamping members; one or moreof each pair of first and second clamping members clamps each fan trackliner panel to the hook 354.

The first clamping member 374 includes a plate 376 and a portion of theseptum layer 362. The septum layer of the fan track liner extends fromthe fan track liner 356 to bridge a gap between the hook and the fantrack liner, and rests on a radially outer surface of the hook. Theplate 376 rests on the septum layer. As can be seen more clearly inFIGS. 7 and 8, a cut-away 384 is made in a radially outer surface (i.e.a non-gas washed surface) of the fan track liner to accommodate theplate 376. In the present embodiment the cut-away and the plate aresubstantially rectangular with rounded corners, but as will beappreciated by the person skilled in the art the plate and cut-away canhave any suitable shape and the shape of the plate need not be the sameas the shape of the cut-away. Provision of a cut-away can reduce oreliminate the protrusion of a fastener of the clamping arrangement(described later) from the nominal profile of the radially outer surfaceof the fan track liner so as to limit any impact of the fastener on theoperation of the containment system in a fan blade off scenario.

In the present embodiment the plate 376 is bonded to the septum layer362, e.g. using an adhesive such as epoxy resin. Adhesive is also usedto fill a portion of the exposed honeycomb structure of the intermediatelayer 360 for improved sealing and strength.

In the embodiment of FIGS. 3 to 8, the second clamping member 372 takesthe form of a plate, more specifically a bent (or angled) plate (theplate may be formed by bending or by an alternative manufacturing methodsuch as extrusion or moulding). The plate is bent so as to be positionedmore radially outward at a position adjacent the fan track liner than aposition adjacent the hook. The plate is further bent to form a lip 373that rests on a radially inner surface of the hook, in the presentembodiment a gas washed surface of the hook.

In the present embodiment, the hook 354 includes an undercut 371 thataccommodates the lip 373 of the second clamping member 372.

The plate of the first clamping member and/or the second clamping membercan be made from any suitable material, including a composite materialor a metal material.

The first clamping member 374 and the second clamping member 372 areconnected together using a fastener 378. The fastener can be tightenedto adjust the compressive force applied to the hook 354 by the clampingarrangement 370. The fastener 378 is axially spaced from the hook 354 tobe positioned between the hook 354 and the fan track liner 356, in thisway no holes or other formations are required in the hook to accommodatethe fastener 378.

Referring to FIG. 8, the fastener 378 of the present invention includesa bolt 381 and a nut 380. The nut 380 includes a plate member that abutsthe plate 376 of the first clamping member 374. A further two rivets 382connect the nut 380 to first clamp member 374 to limit rotation of thenut, this type of nut is often referred to as a basket nut. However, aswill be appreciated by the person skilled in the art, the fastener maytake many different forms, e.g. any type of bolt or screw may be usedwith any type of nut and optionally any type of known washer (forexample a bolt and lock nut may be used). Alternatively or additionallyan adhesive may be used instead of a fastener (e.g. the first and secondclamping members may be bonded together). Further alternatively a clampmay be used instead of a fastener.

A filler material, e.g. the same material as used for the attritionlayer 358 can optionally be provided on a gas washed surface of thesecond clamping member 372 and fastener 378. The filler may extend toprovide a substantially smooth gas washed surface from the hook 354 tothe fan track liner 356. Preferably, any steps in the profile of the gaswashed surface of the fan containment system will be radially outward inthe direction of flow through the gas turbine engine 10.

As mentioned previously, the clamping arrangement 370 includes aplurality of plates 376 of the first clamping member 374, a plurality ofplates of the second clamping member 372, and a plurality of fasteners378 spaced circumferentially around the fan track liner 356 and hook354. The spacing of the plates and fasteners, the dimensions of theplates, and the amount of overlap of the plates with the hook can beselected to optimise fuse load to operate the trapdoor and to optimisetrapdoor vibration frequencies. In an exemplary embodiment, the numberof clamping plates provided may be selected to suit the number ofarcuate fan track liner panels provided around the fan case. Forexample, in the case of there being sixteen fan track liner panels,there may be provided four clamping plates of equal length, eachextending across a respective set of four fan track liner panels. Inorder to facilitate easy maintenance of the fan track liner panels, theends of each clamping plate may coincide with the edge of a fan trackliner panel.

During normal operation of the gas turbine engine 10, the clampingarrangement 370 fixes the position of the fan track liner with respectto the hook in both a radially inward and a radially outward direction.In this way, the fan track liner can resist ice impact and maintainaerodynamic efficiency.

In the event of a fan blade 40 (or part of a fan blade as the case maybe) being released from the hub, the fan blade travels rapidly outwardsand forwards in an axial direction. As the fan blade travels outwards itimpacts the fan track liner 356. Impact of the fan blade with the fantrack liner causes the second clamping member 372 to fail; that is thelip of the second clamping member either bends to move away from thehook, or shears or fractures so as to no longer be in contact with thehook. The failure of the second clamping member means that the fan trackliner 356 is free to move into the voidal region 359 under the forceapplied by the released fan blade. The fan blade then has asubstantially unimpeded path to the hook 354. The fan blade 40 impactsthe hook and is held by the hook 354 and further axially forwardmovement is prevented. A trailing blade then forces the held releasedblade rearwards where the released blade is contained.

The use of the clamping arrangement instead of fasteners of the priorart mitigates the risk of the integrity of the hook being damaged whenthe fan blade impacts the fan track liner, so that the likelihood ofcontainment of a released fan blade is improved. This is because unlikeconventional methods of connecting the fan track liner to the hook, thepresent embodiment does not require holes to be formed in the hook toreceive a fastener. Tests carried out on the presently describedembodiment have found that the clamping arrangement does not interferewith the movement of the fan track liner towards the annular casingelement in a fan blade off scenario. The tests further demonstrated thatvibration integrity is maintained during normal operation of the engine10.

A fan track liner 456 and clamping arrangement of an alternativeembodiment is illustrated in FIG. 9. In the embodiment of FIG. 9features are labelled with similar reference numerals as the previouslydescribed embodiment but with a prefix “4” instead of “3”. Only the maindifferences will be described here.

Instead of using a cut-away to form the recess, as described in theprevious embodiment, the fan track liner 456 of FIG. 9 includesdepressions 484 to form a recess in the fan track liner. Each depression484 is dimensioned so as to accommodate a fastener 478 that connects thefirst and second clamping members 474, 472. The depressions are spacedcircumferentially around the fan track liner 456.

In the embodiment of FIG. 9, the first clamping member 474 includes theseptum layer 462 and the tray 457; the tray 457 providing a similarfunctionality as the plate 376 of the previously described embodiment.

The second clamping member 472 is a flat plate. One end of the platesits in a recess 492 formed in the attrition layer 458 of the fan trackliner, and the other end of the plate rests against a relieved portion(i.e. the undercut) of the gas washed surface of the hook (not shown inFIG. 9).

In the event of a fan blade off event, the second clamping member 472will fail, in a similar manner to that previously described, so as toencourage containment of a released fan blade.

In the embodiments previously described, the second clamping member isprovided by a series of circumferentially spaced plates, as illustratedin FIG. 10A. However, in alternative embodiments, the second clampingmember may form a substantially annular member, for example as shown inFIG. 10B a second clamping member 572 may be provided as a single or astwo (or more) arcuate plates that connect together to form asubstantially annular ring. The substantially annular clamping membermay have a constant profile in an axial direction, but alternatively theclamping member may have a curved profile, e.g. may have a waved profilein the axial direction. An example of a second clamping member 672having a waved profile is shown in FIG. 10C. The waved profile is suchthat the axial width of the clamping member 672 is greater at theposition of each of the fasteners 678 that connect to the first clampingmember (not shown). Filler material 688 can be provided in a regiondefined by a rearward face of the hook (e.g. a forward face of a recessformed between the hook and the annular clamping member) and a forwardface of the annular clamping member 672.

A further alternative fan containment system is indicated generally at700 in FIG. 11. Similar reference numerals as used for the previouslydescribed embodiment of FIGS. 4 to 8 are used for the embodiment of FIG.11, but with a prefix “7” instead of “3”. Only the differences will bedescribed here.

The clamping arrangement 770 of the containment system 700 includes afirst clamping member 774 and a second clamping member 772, similar toas previously described. However, fan track liner 756 includes athreaded insert 790. The insert 790 is bonded in to the attrition liner758 and the intermediate layer 760 through the septum layer 762. Thesecond clamping member 772 is attached to this insert with a screw 781(but in alternative embodiments an alternative type of fastener may beused). If a released fan blade impacts the fan track liner, the secondclamping member 772 fails to permit movement of the fan track liner 756towards the fan case 752, similar to that described previously.

The first clamping member 774 is defined by a portion of the tray 757and the septum layer 762 of the fan track liner.

A yet further alternative fan containment system is indicated generallyat 800 in FIG. 12. Similar reference numerals as used for the previouslydescribed embodiment of FIG. 11 are used for the embodiment of FIG. 12,but with a prefix “8” instead of “7”. Only the differences will bedescribed here.

The main difference between the embodiment of FIG. 12 and the embodimentof FIG. 9 is the shape of the second clamping member. In the embodimentof FIG. 12, the second clamping member 872 includes a plate section anda radial section, and the radial section has a radial thicknesssubstantially equal to the thickness of the attrition layer 858, e.g.the radial section extends from a plate section to the septum layer 862.A radially outer surface of the second clamping member abuts against theseptum layer 862.

The tray 857 of the fan track liner 856 may be formed to includedepressions (indicated by the dotted profile) or the fan track liner 856may not include any depressions (as indicated by the solid line). Whenthe tray does not include any depressions, the second clamping membermay be connected to the tray using a fastener 880, e.g. nut and bolt, oralternatively the second clamping member may be connected to the trayvia the septum layer, e.g. by bonding.

A still further alternative fan track liner and clamping arrangement isshown in FIG. 13. Similar reference numerals as used for the previouslydescribed embodiment of FIGS. 4 to 8 are used for the embodiment of FIG.13, but with a prefix “9” instead of “3”. Only the differences will bedescribed here.

In the embodiment of FIG. 13, the first clamping member 974 is definedby a portion of the septum layer 962 and a plate 976, similar to theembodiment of FIG. 4. The second clamping member 972 is substantiallyL-shaped and includes a support (or radial) portion and a plate portion,the support portion having a greater radial thickness than the plateportion. The fan track liner 956 is connected to the hook by virtue ofthe septum layer 962 only, the septum layer extending to overlap thehook 954 and form part of the first clamping member 974. The tray 957does not extend to the hook 954, and instead terminates before the hook954. As such, no recesses (e.g. depressions or cut-outs) are formed inthe tray 957 for accommodating a fastener.

In the event of a fan blade off scenario, the fan containment system ofFIG. 13 functions differently to the previously described embodiments.In the embodiment of FIG. 13, when a fan blade impacts the fan trackliner 956, the septum layer that spans the gap between the first andsecond clamping members and the fan track liner fails in preference tofailure of the second clamping member 972. The released fan blade thenimpacts the hook and is contained in a similar manner to that describedfor the previous embodiments.

Referring now to FIGS. 14 to 17, a further alternative fan containmentsystem is illustrated. Similar features are given a similar referencenumeral as the previously described embodiments but with a prefix “10”.

The fan containment system 1000 of FIGS. 14 to 17 includes a pair ofcooperating pads or shims 1086, 1088 provided between the first clampingmember 1074 and the second clamping member 1072, and axially adjacentthe hook 1054. As shown most clearly in FIG. 15, the shims 1086, 1088are generally arcuate and are circumferentially wedge-shaped. As willthus be noted, the shims 1086, 1088 are thus each circumferentiallytapered in the sense that their radial thickness varies along theircircumferential extent. As will be described in more detail below, theshims 1086, 1088 are provided in cooperating pairs, as illustrated inFIG. 15, at spaced-apart clamping positions around the fan case 1050.

The radially outermost shim 1086 of each pair has an arcuate outersurface which is configured to bear against the radially innermostsurface of the first clamping member 1074. Similarly, the radiallyinnermost shim 1088 of each pair has an arcuate inner surface which isconfigured to bear against the radially outermost surface of the secondclamping member 1072. Both shims 1086, 1088 present a respective arcuatecontact surface for contact with one another. As will be noted, theshims 1086, 1088 are arranged in circumferentially overlapped relationto one another such that the radially thickest end of the outer shim1086 is generally aligned with the radially thinnest end of the innershim 1088 and vice-versa. The cooperating contact surfaces of the twoshims may be roughened so as to have a relatively high coefficient offriction.

Each shim 1086, 1088 is provided with a series of three through-holes1090, 1091 in spaced relation to one another along the circumferentiallength of the shim, and which extend through the radial thickness of theshim. The holes provided through each shim are arranged for generalalignment with the holes provided through the other shim. However, theholes are not identical for each shim. In the case of the radiallyinnermost shim 1088, the holes 1090 are generally circular, whilst inthe case of the radially outermost shim 1086 the holes 1091 are somewhatelongate in the circumferential direction, as illustrated most clearlyin FIG. 16 which shows an arrangement in which the holes 1091 extendingthrough the radially outermost shim 1086 are generally oval.

The shims 1086, 1088 are provided in cooperating pairs around the fancase, at respective clamping positions, and are settable relative to oneanother to ensure that a predetermined clamping load is applied betweenthe first and second clamping members 1074, 1072 when each clampingarrangement 1070 is configured and clamped around the hook 1054. As willbe appreciated, the shims 1086, 1088 of each pair thus cooperate todefine a radial spacer 1092 between the clamping members 1074, 1072 aswill be described in more detail below.

In order to configure the clamping arrangement 1070, the two shims 1086,1088 are provided in overlapping relation as illustrated in FIG. 15,such that their respective contact surfaces are in contact with oneanother. It is envisaged that the actual degree of circumferentialoverlap between the two shims 1086, 1088 will be carefully adjusted, viacircumferential movement of the two shims relative to one another, toset the radial thickness of the radial spacer 1092, which the two shims1086, 1088 define such that it is slightly less than the local radialthickness of the adjacent region of the hook 1054, by a predeterminedoffset distance.

As will be appreciated, the elongate and oval configuration of the holes1091 extending through the outermost shim 1086 allows those holes toremain aligned and in communication with the circular holes 1090 throughthe innermost shim 1088 throughout an appropriate range of relativeadjustment between the two shims. Once the two shims 1086, 1088 of eachpair have been properly adjusted relative to one another in this way,their relative positions are then set by the insertion of a fastener, inthe form of a setting bolt or screw 1094, through the aligned centreholes 1090, 1091 of the two shims 1086, 1088. The setting bolt 1094 maybe threadingly engaged with a setting nut 1079 which is located againstthe radially outermost side of the first clamping member 1074. Asillustrated in FIG. 15, the central hole 1090 in the radially innermostshim 1088 may be countersunk to accommodate the head of the settingscrew 1094.

As will be appreciated, the roughened contact surfaces between the twoshims supplement the function of the setting screw 1094 in preventingrelative movement between the shims once they have been set to theirrequired positions relative to one another. Additionally, or eveninstead of the fixing screw, the shims may be bonded to one another oncetheir relative positions have been set, for example via the use of asuitable adhesive such as epoxy resin.

When the shims 1086, 1088 have been set relative to one another asdescribed above, the second clamping member 1072 can be offered up tothe arrangement and clamped against the radially inner surface of thehook 1054 by the insertion and tightening of a pair of fasteners in theform of clamping screws or bolts 1078 which are inserted radiallyoutwardly through respective apertures 1096 formed in the secondclamping member 1072; through the aligned end holes 1090, 1091 in theshims and through corresponding apertures formed in the first clampingmember. The clamping screws 1078 are threadingly engaged with respectiveclamping nuts 1080 which are located against the radially outermost sideof the first clamping member 1074, on opposite sides to the setting nut1079. As will be noted from FIG. 4, the clamping screws 1078 are spacedaxially from the hook 1054, and do not pass through the hook.

As illustrated in FIG. 17, the radially outward side of the fan trackliner 1056 includes a depression 1084 at each clamping position, to forma recess in the fan track liner 1056. Each depression 1084 is sized toaccommodate the nuts 1079, 1080 of the clamping arrangement. Theprovision of a depression 1084 at each clamping position in this manneravoids the protrusion of the nuts 1079, 1080 from the nominal profile ofthe radially outer surface of the fan track liner so as to limit anyimpact of the nuts on operation of the containment system in a fan bladeoff event.

It should appreciated at this juncture that the three nuts 1079, 1080could be replaced with a single nut member taking the form of a platewhich is riveted to the outer surface of the first clamping member 1074and which defines respective threaded holes or sockets to receive thesetting screw 1094 and the clamping screws 1078.

As the clamping screws 1078 are tightened, the first and second clampingmembers 1074, 1072 are drawn towards one another to clamp around thehook 1054. As indicated above, the shims 1086, 1088 are set relative toone another such that the radial spacer 1092 which they define betweenthe two clamping members is thinner than the radial thickness of thelocal adjacent region of the hook 1054, by a predetermined offset. Thisoffset is carefully selected so that when the clamping members 1074,1072 are clamped around the hook 1054 as described, a predeterminedclamping load is applied to the hook 1054. Additionally, as illustratedmost clearly in FIG. 4, the offset may be selected to ensure that theradially innermost surface of the second clamping member 1072 issubstantially aerodynamically flush with the radially innermost surfaceof the hook 1054 and/or the radially innermost surface of the fan trackliner 1056.

As indicated above, it is intended to provide clamping arrangements 1070of the type described above at circumferentially spaced apart positionsaround the fan case 1050. Each clamping arrangement 1070 may comprise asingle pair of shims 1086, 1088 of the type described above, or mayalternatively comprise two pairs of shims provided adjacent one another.

It is intended that the shims 1086, 1088 of each pair and/or eachclamping arrangement will be set relative to one another as described,independently of each other pair and in dependence on the local radialthickness of the hook 1054. In this manner, the shims can be adjusted toaccount for circumferential variations in the radial thickness of thehook 1054, and thus the shims of each pair may be set to define aslightly different radial thickness than other pairs. Nevertheless, itis intended that the shims 1086, 1088 of each pair will be identical tothe shims of each other pair. Because of their circumferentially taperedconfiguration, the shims of each pair can be identical whilst stillaccommodating sufficient local adjustment of their radial thickness,without needing to provide a large number of differently sized shims.

During normal operation of the gas turbine engine 10, the clampingarrangement 1070 fixes the position of the fan track liner 1056 withrespect to the hook 1054 in both a radially inward and a radiallyoutward direction. In this way, the fan track liner can resist iceimpact and maintain aerodynamic efficiency.

By providing the pairs of wedge-shaped shims between the two clampingmembers, account can be taken of circumferential variations in thethickness of the hook, thereby ensuring substantially equal clampingforce is applied to the hook at all clamping positions.

It will be appreciated by one skilled in the art that, where technicalfeatures have been described in association with one embodiment, thisdoes not preclude the combination or replacement with features fromother embodiments where this is appropriate. Furthermore, equivalentmodifications and variations will be apparent to those skilled in theart from this disclosure. Accordingly, the exemplary embodiments of theinvention set forth above are considered to be illustrative and notlimiting.

The invention claimed is:
 1. A fan containment system for fitment aroundan array of radially extending fan blades mounted on a hub in an axialgas turbine engine, the fan containment system comprising: a fan casehaving an annular casing element for encircling the array of fan bladesand a hook projecting in a generally radially inward direction from theannular casing element and positioned axially forward of the array offan blades when the fan containment system is fitted around the array offan blades; an annular fan track liner positioned substantially coaxialto the annular casing element; and a clamping arrangement connecting thefan track liner to the hook, wherein the clamping arrangement comprisesa first clamping member positioned radially outward of the hook and asecond clamping member positioned radially inward of the hook, the hookbeing clamped between the first clamping member and the second clampingmember, wherein the first clamping member is connected to the secondclamping member via one or more fasteners that are axially spacedrearwardly from all portions of the hook, and wherein the clampingarrangement is configured such that under the condition that a releasedfan blade impacts the fan track liner, the clamping arrangementsubstantially releases the connection between the hook and a portion ofthe fan track liner so that at least a portion of the fan track linercan move towards the annular casing element so as to encourage thereleased fan blade to impact the hook.
 2. The fan containment systemaccording to claim 1, wherein at least a portion of a component of theclamping arrangement is configured to fail when the released fan bladeimpacts the fan track liner.
 3. The fan containment system according toclaim 2, wherein the second clamping member is configured to fail whenthe released fan blade impacts the fan track liner.
 4. The fancontainment system according to claim 1, further comprising a connectorconnecting the first clamping member and the second clamping member tothe fan track liner, and wherein the connector is configured to failwhen the released fan blade impacts the fan track liner.
 5. The fancontainment system according to claim 1, wherein the hook comprises anundercut that accommodates at least a portion of the second clampingmember.
 6. The fan containment system according to claim 1, wherein theone or more fasteners extends through at least a portion of the fantrack liner.
 7. The fan containment system according to claim 1, whereinthe first clamping member comprises at least a portion of a component ofthe fan track liner.
 8. The fan containment system according to claim 1,wherein the fan track liner comprises a tray proximal to the annularcasing element, an intermediate layer connected to the tray, anattrition layer proximal, in use, to the fan blades, and a septum layersubstantially radially between the intermediate layer and the attritionlayer, and wherein the first clamping member comprises a portion of theseptum layer and/or the tray.
 9. The containment system according toclaim 1, wherein the first clamping member and/or the second clampingmember comprise a series of discrete circumferentially spaced plates.10. The containment system according to claim 1, wherein the clampingarrangement further comprises at least one pair of cooperatingwedge-shaped shims positioned between the clamping members and axiallyadjacent the hook, the shims being arranged relative to one another todefine a radial spacer between the clamping members.
 11. A fancontainment system according to claim 10, wherein the shims of each saidpair are arranged relative to one another such that the radial spacerwhich they cooperate to define between the clamping members has a radialdimension which is less than the local radial thickness of the adjacenthook.
 12. A fan containment system according to claim 11, wherein saidradial dimension of each radial spacer defined by the cooperating shimsis effective to position the second clamping member such that a radiallyinnermost surface of the second clamping member is aerodynamically flushwith a radially innermost surface of the hook and/or a radiallyinnermost surface of the fan track liner when the hook is clampedbetween the first and second clamping members.
 13. A fan containmentsystem according to claim 10, the system having a plurality of saidpairs of cooperating shims, said pairs of shims being circumferentiallyspaced-apart around the fan case.
 14. A fan containment system accordingto claim 13, wherein each pair of shims is substantially identical toeach other pair of shims.
 15. A fan containment system according toclaim 14, wherein the shims of each pair are arranged relative to oneanother independently of the shims of each other pair, such that theradial thickness of each radial spacer defined by a respective pair ofshims is set independently to thereby accommodate any circumferentialvariation in the radial thickness of the hook.
 16. A fan containmentsystem according to claim 10, wherein each of the shims of each pair iscircumferentially tapered.
 17. A fan containment system according toclaim 16, wherein the shims of each said pair are configured forcircumferential movement relative to one another prior to the hook beingclamped between the clamping members, to thereby adjust the radialthickness of the radial spacer defined by the shims.
 18. A fancontainment system for fitment around an array of radially extending fanblades mounted on a hub in an axial gas turbine engine, the fancontainment system comprising: a fan case having an annular casingelement for encircling the array of fan blades and a hook projecting ina generally radially inward direction from the annular casing elementand positioned axially forward of the array of fan blades when the fancontainment system is fitted around the array of fan blades; an annularfan track liner positioned substantially coaxial to the annular casingelement; and a clamp having a first jaw opposing a second jaw, the hookbeing clamped between the first and second jaws, wherein the first jawis connected to the second jaw via one or more fasteners that areaxially spaced rearwardly from all portions of the hook, wherein theclamp is connected to the fan track liner or defines at least a portionof the fan track liner, such that under normal running conditions theclamp substantially fixes the radial position of the fan track linerwith respect to the hook, and wherein under the condition that areleased fan blade impacts the fan track liner, the second jaw isconfigured to fail so that at least a portion of the fan track liner canmove towards the annular casing element so as to encourage the releasedfan blade to impact the hook.
 19. A gas turbine engine comprising thefan containment system according to claim 1.